The present embodiments relate to wireless communication systems and, more particularly, to operation of a communication system in which a user equipment (UE) communicates with a base station (eNB) equipped with a large number of antennas.
With Orthogonal Frequency Division Multiplexing (OFDM), multiple symbols are transmitted on multiple carriers that are spaced apart to provide orthogonality. An OFDM modulator typically takes data symbols into a serial-to-parallel converter, and the output of the serial-to-parallel converter is frequency domain data symbols. The frequency domain tones at either edge of the band may be set to zero and are called guard tones. These guard tones allow the OFDM signal to fit into an appropriate spectral mask. Some of the frequency domain tones are set to values which will be known at the receiver. Among these are cell-specific reference signals (CRS), channel state information reference signals (CSI-RS), and demodulation reference signals (DMRS). These reference signals are useful for channel measurement at the receiver. Cell-specific reference signals as well as channel state information reference signals are not precoded and are generated by a pseudo-random sequence generator as a function of the physical cell ID. In Releases 8 through 10 of the Long Term Evolution (LTE) of the Universal Mobile Telecommunications System (UMTS), which was designed for conventional point-to-point communication, the cell ID is not explicitly signaled by the base station (called eNB) but is implicitly derived by the UE as a function of the primary synchronization signal (PSS) and secondary synchronization signal (SSS). To connect to a wireless network, the UE performs a downlink cell search to synchronize to the best cell. A cell search is performed by detecting the PSS and SSS of each available cell and comparing their respective signal quality, for example, in terms of reference signal received power (RSRP). After the cell search is performed, the UE establishes connection with the best cell by deriving relevant system information for that cell. Similarly, for LTE Release 11 the UE performs an initial cell search to connect to the best cell. To enable multi-point CoMP operation, the connected cell then configures the UE by higher-layer signaling with a virtual cell ID for each CSI-RS resource associated with each respective base station involved in the multi-point CoMP operation. The UE generates the pseudo-random sequence for each CSI-RS resource as a function of the virtual cell ID.
Conventional cellular communication systems operate in a point-to-point single-cell transmission fashion where a user terminal or equipment (UE) is uniquely connected to and served by a single cellular base station (eNB or eNodeB) at a given time. An example of such a system is Release 8 of the 3GPP Long-Term Evolution. Advanced cellular systems are intended to further improve the data rate and performance by adopting multi-point-to-point or coordinated multi-point (CoMP) communication where multiple base stations can cooperatively design the downlink transmission to serve a UE at the same time. An example of such a system is the 3GPP LTE-Advanced system. This greatly improves received signal strength at the UE by transmitting the same signal to each UE from different base stations. This is particularly beneficial for cell edge UEs that observe strong interference from neighboring base stations.
Most UEs which communicate with a single eNB are configured with a single CSI-RS resource. Other UEs may be configured for CoMP where multiple eNBs coordinate with each other in servicing the UE. In particular, DL transmission from multiple adjacent eNBs is coordinated to avoid or cancel inter-cell interference. This effectively reduces interference and boosts the signal-to-noise ratio at the UE. One example of CoMP transmission is joint processing, where data for a single UE might be transmitted from multiple adjacent eNBs. A UE receiving CoMP transmission, therefore, needs to be configured with multiple CSI-RS resources in order to measure respective channels of multiple eNBs. In this case, each CSI-RS resource is separately configured by higher layer RRC signaling including the CSI-RS antenna port number, a CSI-RS resource index, periodicity and offset of the CSI-RS transmission, and relative transmit power of the CSI-RS.
FIG. 1 shows an exemplary wireless telecommunications network 100. The illustrative telecommunications network includes base stations 101, 102, and 103, though in operation, a telecommunications network necessarily includes many more base stations. Each of base stations 101, 102, and 103 (eNB) is operable over corresponding coverage areas 104, 105, and 106. Each base station's coverage area is further divided into cells. In the illustrated network, each base station's coverage area is divided into three cells such as 104a, 104b, and 104c. A handset or other user equipment (UE) 107 is shown in cell A 104a. Cell A is within coverage area 104 of base station 101. Base station 101 transmits to and receives transmissions from UE 107 over channel 108. UE 107 is configured with CSI-RS resources to measure channel 108 from eNB 101. UE 107 may also receive transmissions from eNB 102. UE 107 is configured by higher layer RRC signaling with separate CSI-RS resources in order to measure channel 109 from eNB 102.
Base stations 101 and 102 configure UE 107 for periodic uplink Sounding Reference Signal (SRS) transmission. Base station 101 estimates channel quality from the SRS transmissions. For downlink (DL) data transmission, UE 107 measures the DL wireless channel from DL reference signals and reports Channel State Information (CSI) to the eNB. The eNB uses the CSI report to perform DL link adaptation and scheduling to determine data transmission schemes to the UE, including time/frequency resource assignment, modulation, and coding schemes.
The DL reference signals used by UE 107 may be Cell-specific Reference Signals (CRS) or Channel State Information Reference Signals (CSI-RS) in LTE. The CSI-RS resource configuration includes a number of CSI-RS antenna ports, a CSI-RS resource index, periodicity of CSI-RS transmission, and relative transmit power of the CSI-RS. CSI is reported in the form of a set of recommended MIMO transmission properties to the eNB. CSI includes a Channel Quality Indicator (CQI), precoding matrix indicator (PMI), and rank indicator (RI). RI indicates the number of data layers that the UE recommends the eNB to transmit. PMI is the index to a recommended precoding matrix in a predetermined codebook known to the eNB and the UE. CQI reflects the channel quality that the UE expects to experience if the recommended RI and PMI are used for data transmission. The time and frequency resources that can be used by the UE to report CSI are controlled by the eNB. The UE is semi-statically configured by higher layers to periodically feedback different CSI components (CQI, PMI, PTI, and RI) on the Physical Uplink Control Channel (PUCCH). Different PUCCH modes can be configured for CSI feedback.
FIG. 2 illustrates CSI-RS resources in a physical resource block (PRB) pair that can be configured for a UE using 2Tx, 4Tx, and 8TX MIMO, respectively, for an OFDM system with a normal cyclic prefix (CP). These CSI-RS resources allow the UE to perform channel estimation. The number of CSI-RS resources varies according to the antenna configuration. For each channel that the UE needs to measure, one of the available CSI-RS configurations is specified to the UE by higher layer signaling. FIG. 3 is similar to FIG. 2 and illustrates CSI-RS resources in a physical resource block (PRB) pair that can be configured for a UE using 2Tx, 4Tx, and 8TX MIMO, respectively, for an OFDM system with an extended cyclic prefix (CP).
The difference between a physical antenna and an antenna port is herein described for the multi-vendor LTE system. Different eNB vendors may deploy different numbers of physical antennas at their eNB products. Furthermore, the number of physical antennas for different types of base stations may be different. For example, a macro base station designed for wide area coverage may deploy a large antenna array, while a small form-factor base station (e.g. a pico- or femto-cell base station) that is designed to cover a relatively small area may deploy a small number of physical antennas. In order to limit standardization efforts while allowing sufficient implementation flexibility for eNB vendors, LTE has adopted the “antenna port” concept. An antenna port is a reference signal on which the wireless propagation channel property experienced by one signal can be inferred by another signal. As such, an antenna port is uniquely determined by a reference signal from which the UE can measure the associated channel. Hence, if two physical antennas are used to transmit the same reference signal, they appear to a UE as one antenna port. In this case, the UE is not able to differentiate between these two physical antennas. The mapping between physical antennas and antenna ports is determined by the eNB and may be transparent to the UE. Therefore, the UE can differentiate between different antenna ports, because they are associated with different reference signals, but it cannot differentiate between different physical antennas.
While the preceding approaches provide steady improvements in interference measurement and Channel State Information reporting for wireless communications, the present invention is directed to further improvements. Accordingly, preferred embodiments described below are directed toward this as well as improving upon the prior art.